chasing an amp fault in this solid state Marshall

[Brian Roth]

Yes c10. Typo.


Further testing measuring pin5 of ic 4 and get clean and boost channels without switching. So I can adjust level changes by adjusting gain on clean and boost.

?????? That is totally wrong. Switching should allow only ONE pathway through ONE set of volume controls and tone stacks at any given time. Switch to clean, boost controls have no effect and vice versa.

Pin7 of ic 4 yields -35(od1) and -18o(od2) Dani respectively,

This is consistent with previous measurements and the level drop at R22.

 

[Brian Roth]

What DC voltages do you see (referenced to ground/0V) on pin 1 of each 5201 as you switch between clean and boost?

Bri (glad I have a stash of straws to grasp at)

 

[rh001]

5201 should only allow one of it's opamps to pass signal at any time. by your measurements prob does seem to be in the vicinity of ic4. ic4A has unity gain. ic4B has gain of 2.4. Check to see that you're getting such gains. Also look at switching pin1...does it swing all the way from On voltage to OFF voltage? A partial swing could be the problem. If swing is correct then switch inside ic4 is likely broken.

 

[rh001]

Correction: ic4A has gain of 2.. ic4B has gain of 3.4.

 

[Admiral-dk]

As far as I remember - you will NOT see Pin1 change very much voltage vise when you change between the two halves (that has caught me a few times).

 

[james-finetune7]

Maybe someone replaced the 5534 in the past for a 741 trying to fix this same problem and failing. How's the power supply for IC1 looking? Lot of caps here, any of them passing DC? The twin T "contour" circuit could have a resistor that went haywire. It could read okay when the amp is off, but maybe once current is applied it's resistance increases. Maybe. I've seen it happen. My money is on a bad opamp.

I'd have to agree on the op-amp replacement being the problem. No designer would spec a 5534 when a much cheaper Tl071, or an archaic 741 would do the job. The NE5534 from National Semiconductor is designed to be able to source much higher output loads, e.g. the following stage(s). Either Mouser or Digikey.com source the 5534 for a reasonable price in low quantities (1-10 units).